Stress is related to a force imposed on a material or body and strain is the deformation of the material or body responsive to that stress force. The force that produces the stress can be compressive or tensile. Stress is defined asσ=F/A where σ is the stress, F is the force imposed on the material or body and A is the area over which that force is exerted.
The relationship between the stress and the resulting strain as manifested by a specific material is referred to as the stress-strain curve for that material. The curve is unique for each material and relates the amount of deformation (strain) at various values of tensile or compressive loading (stress). These curves reveal many of the properties of a material, including data to establish the materials modulus of elasticity (Young's modulus).
Strain is a dimensionless quantity that is a measure of body deformation representing the displacement of particles in the body relative to a reference length or another reference dimension. Strain measures are usually expressed as a percent or a decimal fraction of the reference dimension when no stress forces are present. For example, ΔL/L is a ratio indicating strain, where ΔL is a measure of a change in a body dimension (deformation due to compression for example) and L is a measure of the body dimension when no stress forces are present.
A passive SAW (surface acoustic wave) device comprises a transducer that generates an acoustic wave in response to an input signal, for example an interrogation signal. The waves propagate on the surface of a material (referred to as a substrate and which may comprise lithium niobate, for example) to a reflector array. The acoustic waves reflect from the reflector array back to the transducer where they are received and processed. The characteristics of the reflected wave are responsive to physical parameters of the reflector array. For example, spacing of the reflectors or elements of the reflector array are affected by a temperature of the material, which may in turn be affected by an ambient temperature of the region surrounding the SAW device.
Characteristics of the reflected waves (e.g., time delay, propagation losses, phase delay) indicate certain characteristics of the substrate or a material to which the substrate is affixed. These characteristics may include temperature, forces exerted, and resulting stresses. As the spacing of the reflector array elements changes the frequency of the reflected wave, either primary or secondary, may also be affected. Displacement can be measured in this way.
FIG. 1 depicts a prior art SAW device 10 (also referred to as a SAW sensor). An interrogating signal comprises a radio frequency (RF) signal pulse 12 transmitted by an RF transceiver 14. The interrogating signal is received by an antenna 18 connected to an interdigital transducer (IDT) 20 disposed on a piezoelectric substrate 24. The IDT 20 launches an incident surface acoustic wave (SAW) 28 onto the piezoelectric substrate 24 in response to the received interrogating signal. The transmitted wave travels along the surface of the piezoelectric substrate 24 as illustrated in FIG. 1.
The SAW 28 propagates along the substrate 24 and is received at a reflector array 30 also disposed on the piezoelectric substrate 24. The reflector array 30 comprises a pattern of metal electrodes (also referred to as elements) that impart an impulse response to the incident SAW 28. The impulse response of the reflector array 30 is imparted to the incident SAW 28 as it launches a reflected SAW 34 back to the IDT 20.
The IDT 20 receives and converts the reflected SAW 34 to an electrical signal that is then radiated from the antenna 18 back to the RF transceiver 14 for extraction of the desired information in the reflected signal.
A SAW device can sense piezoelectric crystal strain as the strain modifies the reflector array 30 and thus the reflected signal. For example, either the frequency shift of the reflected signal or the time delay of the reflected signal can be measured as an indication of the strain.
This technique provides a wireless strain sensor that can be mounted onto translating or rotating components where wire or other physical connections are not practical.
One fundamental difficulty associated with using a conventional SAW device to measure strain is the very small strain-to-failure property for LNB (lithium niobate) and similar piezoelectric substrates on which SAW devices are fabricated. The elastic strain limits of the piezoelectric crystals are typically about 0.1%. Many metals can withstand (that is, without plastic deformation) strains of up to about 0.2% in the elastic range. Other metals and certain materials, especially composites, can withstand strains in excess of about 0.5% in the elastic range. Elastomeric materials can withstand stress forces inducing strains exceeding 200%.